The present invention concerns quasi-ballistic transport of electrons in high resistivity semiconductors or insulators, when exposed to small (around 100 V/cm) electric fields. Quasi-ballistic transport means that electron scattering is reduced to a minimum so that the electron mean free path becomes macroscopic. This effect has so far only been detected in semiconductors when very large electric field strengths are applied over very short distances and/or the semiconductor is cooled down to very low temperatures. A semiconductor or insulator material with the above properties will hereafter be mentioned Quasi-Ballistic Semiconductor or QB-Sem.
The quasi-ballistic transport can be utilised in a number of ways. In this application these will be separated into two major fields:    1. Electron transmitting semiconductors, where it is the transport properties of the quasi-ballistic electrons inside the material which are the characteristic property, and    2. Electron sources, where it is the property of quasi-ballistic electrons to be emitted from the substrate which are the characteristic property.
None of the relevant prior art mentions ballistic electrons in highly resistive semiconductors or insulators, neither when exposed to large electrical fields. This fact is due to the general opinion of quasi-ballistic transport in semiconductors. The possibility of quasi-ballistic transport in high resistive materials is counter intuitive and have therefore never been sought for so far. The understanding of the essential physics of this quasi-ballistic transport process have been that, as long as the applied electrical field E is within ohmic range (mobile charge carriers' concentrations and electrical mobility are constant and independent of the electric field E) and the thickness of the said piece of semiconducting or insulating material is larger than the mean free path of the mobile charge carriers (at best of the order of some one to two thousand Angstroms), then the current component from ballistic electrons is negligibly small, leading to essentially zero value of electron emission. (For references, see S. M. Sze: Physics of semiconductor devices; John Wiley 1981 or K. W. Boer: Survey of semiconductor physics, vol. II; Van Nostrand Reinhold 1992)
1. Electron Transmitting Semiconductors
The quasi-ballistic transport of electrons in high resistivity semiconductors or insulators, when exposed to small (around 100 V/cm) electric fields, is a property, which can be used in more or less any semiconductor component or device.
Semiconductor components and devices cover a vast field of applications and the patents and references within the area are numerous. Four major classes of applications have been made with examples of products in each class.
Class A: Rectification and Charge (Information) Storage.                Semiconductor components/devices in this class include Schottky barrier diodes (U.S. Pat. No. 5,627,479 and EP672 257 B1), bipolar p-n, p-i-n diodes, thyristors as well as a number of unipolar devices such as MIS (Metal-Insulator-Semiconductor) diodes, CCD (Charge-Coupled Devices), MIS tunnel diodes, MIS switch diodes, IMPATT (Impact Ionisation Avalanche Transit Time) and BARITT (Barrier Injection and Transit Time) diodes and other related Transit Time devices.        
Class B: Photo-Sensing and Photo-Emitting Devices                This class of semiconducting components/devices include among others LEDs' (Light Emitting Diodes), Photodiodes, Semiconducting Lasers, Avalanche diodes and other photoconducting devices for light to electrical signal conversion purposes.        
Class C: Amplification and Non-Volatile Memory                Applications of the present invention in this class of semiconductor components/devices include bipolar transistors and bipolar unijunction transistors, together with a number of unipolar components and devices inclusive FETs (Field Effect Transistor), JFETs (Junction Field Effect Transistor), MESFETs (Metal-Semiconductor Field Effect Transistor), MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistor) and Non-Volatile Memory devices. Particularly relevant in relation to present invention within this class are tunnel transistors, TEDs (Transferred-Electron Devices) and other ballistic (Hot Electron) transistors and/or devices.        
Class D: Optical Image Detection, Formation and Processing                Semiconductor camera, Conversion of electrical signals to 2D-optical images/signals, 2D-optical image/signal brightness/contrast amplification and spatial magnification.        
Ballistic or hot electron devices as they are sometimes called, have been anticipated (see for example S. M. Sze: Physics of semiconductor devices; John Wiley 1981, p. 184, but also K. W. Boer: Survey of semiconductor physics, vol. II; Van Nostrand Reinhold 1992, p. 1265 and 1247), but the proposed structures are costly to produce and unreliable, requiring extremely small dimensions (of the order of one hundred Angstroms) and high electrical fields.
2. Electron Sources
The present invention relates to a general class of electron devices termed “electron sources” and more specifically to a subclass termed “planar electron sources”. All of these devices provide a beam of electrons that can move through the empty space and be used for various technological applications.
The essential requirement for all electron sources is to provide sufficient amount of electrons at the emitting surface of the device (the surface of the device facing the vacuum) with sufficient amount of energy (3-5 eV in most cases) and a velocity in the direction of emitting surface in order that these electrons can surmount the energy barrier at the emitting surface—vacuum interface and escape from the material into vacuum. The energy barrier is roughly given by the energy difference between the vacuum level and the electron chemical potential at the emitting surface. The necessary amount of energy can be supplied by any of the following means:                Heating the emitting surface (“Thermal emission” electron sources)        Establishing a sufficiently high electrical field in the region emitting surface-vacuum (“Field emission” electron sources)        Sufficient acceleration of electrons within the bulk region of the device in the direction towards the emitting surface (“Tunnelling field emission” and/or “Quasi-ballistic field emission” electron sources).        Illumination of the emitting surface with help of photons or other energetic particles (“Photo emission” electron sources).        Lowering of the said energy barrier at the emitting surface-vacuum interface (“Negative electron affinity emission” electron sources).or by the combination of any of the above methods.        
While for some applications a point source electron beam is required, where the electrons are subsequently accelerated and electro-optically modulated, there are a large number of technological applications, where a planar source of electrons is required and/or would be advantageous. All of the prior art to be used in these applications, relate to small pointlike emission regions from a specific material detail at that point. A larger planar electron emitter can only be achieved by making an array of such small regions. Moreover, most devices need an opening in the anode for the electrons to escape into vacuum.
There is a very large number of inventions, as can be seen for example from citations in the U.S. Pat. No. 5,703,435 (December 1997) and the U.S. Pat. No. 5,534,859 (September 1996), that all relate to planar electron emitters with the main emphasis on the use of these inventions as basic building blocks in field emission Flat Panel Displays.
Most of the prior art can be broadly divided into two classes.
Class 1
In the first class the emitting cathode-anode structures are usually of all solid state construction and are formed from a combination of metallic, semiconducting and insulating materials in order to establish the necessary conditions for the electron field emission to take place at the anode surface-empty space interface. The intentions of these devises are to improve electron emission efficiency, all using the same basic cathode with several substances disposed thereon. Electrons are emitted from the semiconductor surface into free space though an aperture of the anode. The principle is to narrow the semiconductor-free space barrier and to give the electrons the momentum to escape and/or tunnel through the electric potential barrier of an anode. Any of the above mentioned means can be applied in order to increase the electron emission current Iem.
It is a particularly characteristic feature of a majority of solid state devices in class one of the prior art, that the necessary large external voltages have to be applied over relatively very short distances (of the order of the electron mean free path), in order to generate sufficiently strong electric fields that facilitate the generation and the acceleration of electrons. These electrons then travel along what could be called quasi-ballistic trajectories in the said strong electric field (usually undergoing also an avalanche multiplication here) towards the surface of the emitting anode. At the same time however, they loose, on their way, an appreciable amount of energy through inelastic collisions (scattering). The present understanding is that large voltages are needed in order to obtain considerable emission of electrons through this method. If the applied electrical field E is to small (within ohmic range, mobile charge carriers' concentrations and electrical mobility are constant and independent of the electric field E) and the thickness of the semiconducting or insulating material Lsam (FIG. 2) is larger than the mean free path of the mobile charge carriers (at best of the order of some one to two thousand Angstroms), then the electrical current component Ibal is negligibly small, leading to essentially zero value of the electron emission current Iem (FIG. 2).
Some selected prior art of class 1 are commented below, others are referred in the end of the paragraph.
U.S. Pat. No. 5,536,193 Relates to a method of fabricating a field emitter using the steps of; dispersing small pieces of wide band gap material on a substrate, cover it with a metal, etching the metal away until the wide band gap material comes forth, making small peaks for emitting electrons.
U.S. Pat. No. 5,463,275 Describes only electron emitting devises comprising a layered structure of at least three specialty chosen semiconductor materials
U.S. Pat. No. 4,801,994 Relates to a three-layer semiconductor structure, where the middle layer is supposed to be an intrinsic semiconductor, which supposedly should conduct electrons with very low losses.
EP 504 603 B1 Consists of disposing a complex structure of semiconductors with special impurity levels such as to influence the different depletion regions. The description discloses the use of a Schottky barrier metal-semiconductor junction in order to improve emission efficiency.
U.S. Pat. No. 5,554,859, U.S. Pat. No. 4,303,930 and GB 1 303 659 cover areas similar to EP 504 603 B1
Other relevant references are: Metal-Insulator-Metal electron field emitters (Physical Review Letters Vol. 76, 17 (1996), 320), but also electron field emitters containing various forms of diamond-like components (U.S. Pat. No. 5,631,196, U.S. Pat. No. 5,703,435 and the citations there-off).
Class 2
In some cases of the said prior art, the features characterising class one (combination of more or less planar metallic, semiconducting and insulating materials of various thickness) are combined with feature concerning the formation/concentration and shaping of the necessary electrical field. The emitting cathode in this case is usually prepared in order to facilitate electron field emission from a single point. This is obtained either through covering the material with a low electron work function at small local areas and/or shaping the material geometrically to create an emissive point or peak.
Some selected prior art of class 2 are commented below, others are referred in the end of the paragraph.
U.S. Pat. No. 5,229,682 Concerns a field electron emission device, in which electrons enter free space directly from a part of the emitting electrode pointing through an aperture in the opposite electrode and interjacent layer. The electrons are not traversing any interjacent semiconductor or insulator layer. The emitting electrode is shaped in order to have a part that peaks through the aperture in the opposite electrode and interjacent layer. A flat panel display is made by an array of such electrodes.
U.S. Pat. No. 5,712,490 Concerns a photocathode device, comprising several semiconductor layers disposed on a window layer, the semiconductor layers chosen; to optimise the ability to absorb photons, that is photoconductivity, to increase the diffusion length of those electrons. The invention does not disclose an optically transparent electrode to be disposed between the window layer and the first semiconductor layer (see 3rd column, line 11)
U.S. Pat. No. 5,528,103 As U.S. Pat. No. 5,229,682, but also comprising focusing ridges, for the purpose of generating an electrical field causing the electrons emitted from the gate electrodes in between them, to converge into a narrow band, not for absorbing electrons. Moreover these electrodes/ridges have to be conductive (though otherwise stated in col. 7, line 27) in order to serve their purpose.
U.S. Pat. No. 5,212,426 As U.S. Pat. No. 5,229,682, but also comprising an integral control for each electrode (pixel) using built in transistors for controlling the supply of electric charge to each emitting electrode
U.S. Pat. No. 4,823,004 Relates to a device for analysing the ballistic trajectories of electrons through a material, as well as gaining information about the material bulk structure by analysing the ballistic trajectories.
U.S. Pat. No. 5,444,328 Relates to a method for building up high voltage electron emitting semiconductor structures in a way which makes electrically breakdown less probable.
U.S. Pat. No. 5,631,196 As U.S. Pat. No. 5,229,682, but with the emitting electrode being flat, the parts peaking through the aperture in the opposite electrode and interjacent layer being replaced by impurity doped diamond parts as the electron emitting substance.
Other relevant references are: U.S. Pat. No. 4,683,399, EP 150 885 B1, EP 601 637 A1, U.S. Pat. No. 5,340,997 and the citations there off).
Exceptions
Exceptions from above classes include devices in which electrons are emitted into the empty space between a cathode and the anode by applying sufficient electrical voltage between them. The emitting cathode in this case is usually either covered by a material with a low electron work function and/or it is geometrically shaped in order to facilitate electron field emission. An example of such a device is:
U.S. Pat. No. 5,703,435 Concerns a field emission cathode in which the material of the electron-emitting layer comprises either a mixture of graphite and diamond crystallites or amorphic diamond.
Applications
In order to utilise electron transmitting semiconductors and electron sources in applicable devices, several extensions have to be made to the basic components described in the prior art.
The emitted electrons might not have the sufficient energy to serve their purpose and have to be further accelerated. This will typically be carried out by having an “accelerating electrode” at some distance from the emitting surface at a high positive electrical potential thereby accelerating the emitted electrons to higher energies in the interjacent empty space.
For the electron-light conversion purposes, the appropriate “luminophor” materials can be incorporated within the anode structure, the said anode structure being either an integral part of the cathode-anode structure or part of the “accelerating electrode” separated from the cathode-anode structure by a finite empty space.
The applications of electron source devices typically include all forms of electron microscopy, Planar electron beams lithography, electron guns for evaporation of materials, x-ray tubes, electron multipliers (photomultipliers, two-dimensional particle/EM radiation detector arrays), electron beam welding machines, Flat Panel Displays (based on electron field emission), and some fast ballistic semiconductor components and devices.
Lithography Prior Art
A single very important application of the present invention is in the field of Lithography (Microlithography) and more specifically in the field of what has been termed in the literature as Planar Electron Beam Lithography (PEBL). Lithographic steps are essential during the process of Integrated Circuit (IC) production. The lithographic part of IC production consists in principle in repetition of the steps of resist deposition onto the surface of a wafer, of the exposing parts of the resist with radiation (photons, electrons or ions) by a “writing tool”, and finally of resist removal. Optical, x-ray and Electron/Ion Beam Lithographs are the known methods that can, at least in principle, accomplish the necessary lithographic tasks during the IC production. The optical lithography is the standard, well-matured industrial technology; its major drawback is the optical diffraction limit on the smallest features that can be printed. In the further strife for decreasing the size of the IC components and ICs in general, this has to be considered as a major drawback. By using electrons as radiation source, the optical diffraction limit is not present. The schematic diagram of the principle behind the Planar Electron Beam Lithography, used in the prior art, is shown in FIG. 8. It uses a basic structure consisting of cathode 1, a thin dielectric film 24, an electron absorbing template 19 and the anode 4. Electrons quantum tunnel through thin dielectric film 24 and emerge into free space FS through the surface S4 only at places, where the anode is in direct contact with the dielectric film. These electrons are then accelerated and projected onto the wafer with pre-deposited electron sensitive resist layer 6. In H. Ahmed et al (incl. some of the inventors of the present invention) “Proceedings of the Conference on Microlithography”; Cavendish Laboratory, Cambridge 1989, it is shown and demonstrated experimentally how to carry out Planar Electron Beam Lithography in practice. In this prior art, the electron lithographic projection system has been demonstrated using a prior art planar electron emitter. However, the accessible electron emitters suffer from other drawbacks: Planar electron emitters suitable for this purpose can expose entire wafers with one broad beam, but, at the necessary voltages, these planar electron emitters have extremely short lifetimes due to the effects caused by the necessary large fields and short distances.
The present invention offers a solution to this problem.
It is a disadvantage of the existing field emission planar electron devices, that large external voltages have to be applied over relatively very short distances (of the order of the electron mean free path) in order to generate sufficiently strong electric fields that facilitate the generation and the acceleration of electrons.
It is another disadvantage that this requirement of relatively very high local electrical fields over relatively very short distances, together with the quality of the material at hand, lead in its consequence, to shorter electron mean free path (larger scattering rates) that in turn effectively sets the limit on possible physical distances within the said devices that electrons can move through without too appreciable energy losses.
It is a further disadvantage that, due to the above mentioned effects, either only a small portion of these electrons have sufficient energy to escape through the (emitting) surface of the anode into a space next to the cathode-anode structure.
It is a still further disadvantage that these devices in general have quite low electron emission currents Iem (FIG. 2) and high background current Iback (FIG. 2),
It is a still further disadvantage of the existing field emission planar electron devices, that they suffer from shortcomings such as too large electrical power consumption per square centimeter of electron emitting surface,
It is a still further disadvantage that the above mentioned effects results in low electron emission efficiency.
It is a still further disadvantage of the existing field emission planar electron devices that they are often unstable and liable to dielectric breakdown that generally seriously limits their lifetime.
It is a still further disadvantage of said devices that they suffer from frequently overheating due to large energy losses in the critical areas of the said devices (high electric fields over very short distances).
It is a still further disadvantage that scaling-up of these planar electron field emission devices (increasing the electron emitting area of the cathode) poses a severe problem.
It is a still further disadvantage of said devices that they use non-standard expensive materials.
It is a still further disadvantage of the existing field emission planar electron devices, that the constructions are too complex.
It is a still further disadvantage of the planar electron emitters emitting a broad beam suitable for the exposure of wafers in the fabrication of IC's, that they have an extremely short lifetime (less than 30 minutes), which makes them unsuitable for Planar Electron Beam Lithography.
Electron Transmitting Semiconductors
The present invention aims at solving the above mentioned disadvantages by using the existence, under proper operating conditions and in certain simple semiconductor and insulator structures, of quasi-ballistic electrons.
It is an object of the present invention to make available a semiconductor or insulator substrate in which electrons move along quasi-ballistic trajectories when said substrate is subjected to low applied external electric fields (≦100 V/cm). The electrons (quasi-ballistic electrons) move along these trajectories from one side of the substrate (surface S2, FIG. 3) to the other side (surface S4), and are accelerated to energies sufficient to escape into vacuum through the electron emitting surface S4. (From now on said substrate is also referred to as Quasi-ballistic semiconductor substrate—“QB-Sem substrate”).
It is another object of the present invention to make available a QB-Sem substrate in which the quasi-ballistic electrons suffer almost no energy losses and momentum changes while moving through the QB-Sem substrate.
It is a further object of the present invention to make available a QB-Sem substrate in which no heat is generated when the substrate is used for quasi-ballistic transport of quasi-ballistic electrons.
It is a still further object of the present invention to make available a QB-Sem substrate in which quasi-ballistic transport is possible at low (ohmic) electrical fields and can take place over macroscopic distances.
It is a still further object of the present invention to make available a QB-Sem substrate in which electron velocities are not limited by high electrical field mobility saturation effects.
It is a still further object of the present invention to make available a QB-Sem substrate in which the electron behaviour is similar to the behaviour of electrons in vacuum tubes.
It is a still further object of the present invention to make available a QB-Sem substrate, which do not require high electrical fields and extremely small substrate thickness (of the order of one hundred Angstroms).
It is a still further object of the present invention to make available a QB-Sem substrate from which simple design, robust, relatively cheap, high reliability and long lifetime semiconductor components can be produced.
It is a still further object of the present invention to make available a QB-Sem substrate to be used in the field of electron-optical applications.
It is a still further object of the present invention to make available a QB-Sem substrate to be used in the design and production of semiconductor components and devices and Integrated Circuits (ICs).
It is a still further object of the present invention to make available a QB-Sem substrate in which the anomalously low power dissipation by QB-electrons within the QB-semiconductor contributes to the solution of the heat generation problem when high packing densities of components in the ICs are used.
It is a still further object of the present invention to make available a QB-Sem substrate so that the design of “Hot Electron” devices does not have to rely on thin film complicated multistructures that are often unreliable and costly to produce.
It is a still further object of the present invention to make available a QB-Sem substrate, which do not require high electrical fields, so that the degradation of the various semiconductor devices through irreversible dielectric breakdown is essentially eliminated.
It is a still further object of the present invention to provide QB-Sem substrates, which are fully integrable with the existing Semiconductor Technology.
It is a still further object of the present invention to provide QB-Sem substrates, which are fully integrable with the existing Integrated Circuit Technology and Production.
It is a still further object of the present invention to provide QB-Sem substrates from which the design of fast high frequency semiconductor components and devices are under no geometrical constraints.
It is a still further object of the present invention to provide QB-Sem substrates with which new design concepts for semiconductor components/devices and/or physical apparatuses are possible.
It is a still further object of the present invention to provide QB-Sem substrates with life times of the same order of magnitude (or longer than) as the usual Semiconductor Industry products.
The effect that, in QB-semiconductors, electrons that can move quasi-ballistically over macroscopic distances at low applied electrical fields, as disclosed by the present invention, will have a major impact on design and construction/manufacture of many semiconductor components and devices of both bipolar and unipolar variety. These will be used either as single units or as components/parts within Integrated Circuits' architecture.
Electron Sources
It is an object of the present invention to provide electron emitters in which the emitted electrons use macroscopic quasi-ballistic trajectories (these trajectories are many hundreds of microns long) in a piece of QB-Sem substrate subject to low externally applied electrical fields (≦100 V/cm). These electrons (quasi-ballistic electrons), moving along these trajectories from one side of the substrate (surface S2, FIG. 3) to the other (surface S4) are accelerated, increasing thereby their energy, and escape into vacuum through the electron emitting surface S4.
It is another object of the present invention to provide planar electron emitters, which are characterised by very low applied electric fields/voltages.
It is a further object of the present invention to provide planar electron emitters, which are characterised by very low power dissipation.
It is a still further object of the present invention to provide thin (≦1 cm) planar electron emitters with all solid state construction.
It is a still further object of the present invention to provide planar electron emitters in which the macroscopic emitting surface has no subdivisions.
It is a still further object of the present invention to provide planar electron emitters characterised by the simplicity and robustness of the assembly (FIG. 3).
It is a still further object of the present invention to provide planar electron emitters characterised by the self supporting structure of the assembly.
It is a still further object of the present invention to provide planar electron emitters in which there are no limits to geometrical scaling-up of electron emitting surface.
It is a still further object of the present invention to provide planar electron emitters in which the electron emission area is large and only limited by the lateral size of the QB-semiconductor wafer, which is today some 800 cm2 (this limit can be of course be overcome by building modules).
It is a still further object of the present invention to provide electron emitters suitable for Planar Electron Beam Lithography
The number of technological applications of the planar quasi-ballistic electron emitter is very large and it is the intention of the authors of the present invention to claim also the use of the present invention in these. These applications include methods and apparatuses/products such as Planar electron beam lithography, Field emission Flat Panel Displays, High speed (low-dissipation) signal transmission devices, High efficiency detectors, efficient Light sources, Electron emission microscopy, Two-dimensional electromagnetic radiation and/or particle detector arrays, High speed, easily integrable semiconductor components, Semiconducting devices using ballistic electrons, variety of (novel) electron sources and many others.